OCNG Ch.2 (1)

OCNG Ch.2 (1)

1. Fit of the Continents

   • Continents seem to fit together like a jigsaw, particularly Africa & South America
   • Alfred Wegener proposed (in1912) that continents were once joined together in one landmass he name Pangaea
   • Surrounded by one ocean called Panthalassa & several smaller sea such as the Tethys Sea
   • Fit not perfect, but assumed gaps & overlap due to erosion and deposition

2. Computer fit of continents by Edward Bullard in 1965

   • Shallow parts of ocean floor close to shore are part of continents
   • Best fit using a depth of 2000 m below sea level to define continents
   • This depth represents the true edge of continents

3. Matching sequences of rocks & mountain chains

   • Sequences of rock on one continent match up with sequences on other continents even though separated by an Ocean
   • Mountain chains which terminate on edge of continents continue on the opposite continent
   • Ex. Appalachian mountains in North America match up with Caledonian mountains in Europe

4. Glacial Ages

   • Evidence of glaciation 300 million years ago in tropical areas of India, Australia, Africa, South America
   • Now, continental sheets only occur in polar regions of Greenland and Antarctica
   • Therefore, either global ice age 300 million years ago or some continents were much closer to the poles than they are today
   • Coal deposits in Europe and & North America indicate vast semitropical swamps 300 million years ago; this undermines the hypothesis of a global ice age 300 million years ago

5. Distribution of Organisms

   • Many extinct organisms found in the fossil record are found on different continents, but probably would not have had the ability to cross oceans
   • Ex, Mesosaurus is found in eastern Africa & Western South America. if able to swim across an ocean, why not more widely distributed?

6. Objections to Continental Drift

   • Wegener published his ideas in 1915 as The Origins of Continents and Oceans
   • His ideas received hostile criticism until his death in 1930
   • How did continents move? Seemed contrary to the law of physics
   • Wegener didn't have any information about the seafloow

7. Continental drift evolves into the Theory of Plate Tectonics

   • The lithosphere is not continuous, it is composed of thin rigid plates. Plates are made of either continental crust, oceanic crust or both
   • Plates can move horizontally with respect to one another

8. Evidence for plate tectonics came from:

   • Mapping of the sea floor from the 1940s onward
   • Study the Earth's magnetic field during the 1950s
   • Deep-sea drilling programs started in 1960

9. Paleomagnetism

   • Magnetite like tiny compass needles which record strength & orientation of the Earth's magnetic field when rock formed
   • Magnetic dip is how much a magnetite particles points into the Earth & can be used to determine the latitude of a rock when it was formed
   • Paleomagnetism is the study of magnetite particles to determine N/S direction & latitude when the rock was formed
   • Igneous rocks solidfy from magma underground or after volcanic eruptions at the surface from lava
   • Nearly all igneous rocks contain magnetite, a naturally magnetic iron mineral
   • Magnetite minerals align themselves up with Earth's magnetic field in fluid magma or lava
   • Magma or lava cools & solidifies, magnetite particles trapped in position & record the angle of Earth's magnetic field at that time
   • In sedimentary rocks a similar process occurs, magnetite particles will align themselves if the sediment is laid down under water

10. Polarity Reversals

    • Polarity of Earth's magnetic field has reversed periodically through geological time, don't understand why
    • Time of reversals can be estimated using radiometric age dating
    • Magnetic polarity has switched about 5 times every million years

11. Sea Floor Spreading

    • deep water mapping program off Oregon & Washington in 1955 using a towed magnetometer
    • Survey showed above average & below average stripes of magnetism on either side of undersea mountains
    • Alternating stripes called magnetic anomalies
    • Harry Hess (1906-1969) was a geologist & US naval captain in WWII
    • Left depth recorder on ship at all times
    • Showed mountain ridges in center & narrow trenches at edges of Ocean basins
    • Published his ideas in book called "History of Ocean Basins" in 1962
    • Proposed sea floor spreading w/ convection cells as the driving force
    • Mid-Ocean Ridge - continuous volcanic underwater mountain range that winds through every ocean basin
    • Rocks of the same age run parallel to the mid-ocean ridge
    • Earth 4.6 billion years old, but oldest ocean floor only 180 million years old
    • Rockets get older moving in a direction perpendicular to the mid-ocean ridge
    • Sea floor spreading occurs along mid-ocean ridge, where new oceanic crust is created
    • Sea floor spreading explained alternating of magnetic stripes in the rocks of ocean basins
    • Oceanic crust destroyed at the deep ocean trenches which are subduction zones
    • Ocean floor acting as a conveyer belt pushing continental plates around during sea flow spreading
    • Radiometric dating of rocks collected by ocean drilling shows that the youngest rock in the ocean is by the mid-ocean ridges
    • Oldest rocks on land 4 billion years old

12. Major Earthquakes along Plate Boundaries

    • Earthquakes caused by fault movement of volcanic eruptions
    • Most large earthquakes occur along ocean trenches, reflecting energy released during subduction
    • Plates moving an average of 2-12 cm per year
    • Rate of 8 cm per year would mean traveling 80 km (~50 miles, half way from College Station to Houston) after a million years